Courgette: white powdery coating on leaves

What it is likely to be on courgette

1. Powdery mildew

   Candidate 1very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Powdery mildews
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — powdery mildew

   Checked 2026-08-13

   A white, floury coating on the upper leaf surface, most familiar on courgettes and cucumbers late in the season. Unlike most fungal diseases it does not need the leaves to be wet — it needs the roots to be dry and the air to be humid, which is why it arrives in a warm dry August.

   Points towards it

   • · the fungus grows on the surface rather than inside the leaf, so it looks and behaves like dust and can be rubbed off with a thumb
   • · it starts on the oldest shaded leaves in the centre of the plant and works outward, which is the reverse of most foliar diseases
   • · powdery mildew wants humid air and dry leaves, which is why it arrives in a dry August rather than in a wet one
   • · it appears on schedule in late summer without anything having happened

   Argues against it

   • · a felted or downy growth rather than a dusty one, especially on the underside, is the other mildew and needs different advice
   • · damage confined to the newest leaves does not match a fungus that starts on the oldest

   Check this next

   Rub the patch with a thumb and then look at the same spot from underneath. Powdery mildew wipes off as dust and leaves green tissue below, sits on the UPPER surface, starts on the oldest shaded leaves near the crown and spreads outward. If it will not rub off, or the underside carries a grey-purple felt beneath a yellow angular patch on top, it is downy mildew and the advice reverses. On a cucumber under glass, also check the silvery-white patterning along the veins of some varieties — that is natural leaf marking and rubs off nothing because it is inside the leaf.

   What to do now

   Water the soil generously and never the leaves, because powdery mildew is a symptom of a plant that is short of water at the root in humid air rather than of a wet leaf. Space the plants and cut out the oldest, worst leaves from the centre to get air moving through, mulch to hold soil moisture, and keep picking — a courgette carrying mildew from mid-August will usually still crop until the frosts, so removing the plant is rarely the answer. Under glass, ventilate day and night.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Powdery mildews — control
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — mildew management

   Checked 2026-08-13

2. Downy mildew

   Candidate 2common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Downy mildews
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — downy mildew

   Checked 2026-08-13

   Pale yellow patches on the upper leaf surface with a greyish or purplish furry growth directly beneath them, the patches often bounded by the leaf veins into angular blocks. It is routinely confused with powdery mildew and behaves in the opposite way: downy mildew needs wet conditions, powdery mildew thrives in dry ones.

   Points towards it

   • · the fruiting growth is felted rather than dusty and appears under angular patches bounded sharply by the leaf veins
   • · unlike powdery mildew it needs free water on the leaf and long humid nights, so it follows wet weather rather than dry
   • · the lower shaded leaves that stay wet longest go first
   • · a wet outdoor patch and a badly ventilated tunnel both hold the leaf wetness it needs

   Argues against it

   • · a dusty coating that rubs off is the other mildew, and the advice differs
   • · it cannot establish without leaf wetness, which a hot dry spell removes

   Check this next

   Turn the leaf over and look at the shape of the patch from above. Downy mildew makes yellow patches on the upper surface with straight edges, because the growth stops at the veins and the patch ends up angular rather than round — and directly beneath each patch, on the underside, is a grey-purple felt visible best first thing in the morning. It does not rub off. Powdery mildew is the opposite on both counts: dusty, on top, and it wipes away with a thumb.

   What to do now

   Get the leaves dry rather than the soil, which is the reverse of the powdery mildew answer. Water the ground in the morning only, never overhead and never in the evening, open every vent and door on a tunnel day and night in warm weather, and take out the lowest leaves so air moves under the canopy. Remove and bin badly affected leaves rather than composting them. Where it has taken hold late in the season the practical answer is to keep picking; where it arrives in July, a later sowing in a different part of the plot often outruns it.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Downy mildews — control
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — downy mildew management

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on courgette — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general white powdery coating on leaves page lists causes across all plants, and the Courgette profile lists the problems it is prone to.